seem to have little foundation in the sources. It has not been possible to show empirically that an increased stratification of society in fact took place at all in this period. Indeed, recent studies seem to indicate that this was not the case. Knut Helle emphasises the steady strengthening of royal power throughout the civil war era. When the period ended, the concept of a unitary kingdom (as opposed to power-sharing) had been accepted, the beginnings of a centralized administration had appeared and the king's power had increased so that a strong king would be able to contain social and geographical splits without them leading to open war. In this perspective the civil wars can be seen as the final phase in the unification of Norway into one kingdom. Sources The main sources for the civil war era are the kings' sagas. Heimskringla, Fagrskinna and Morkinskinna all describe the period up to the year 1177, although the parts of Morkinskinna that are preserved only extend to 1157. These three sagas were written c. 1220–1230, and in using them as historical sources, it has to be remembered that they were thus written a fair amount of time after the events they describe. However, they are likely to have been based on earlier works, in particular the saga Hryggjarstykki, written c. 1150, which is lost to us, but was available to the authors of the three aforementioned sagas. Ágrip af Noregs konunga sögum also describes the civil war era, but has only been preserved up to the events of c. 1136. The period 1177 to 1240 (and beyond) is treated in detail in contemporaneous sagas: Sverris saga (from 1177 to 1202) the Bagler sagas (1202 to 1217) and Håkon Håkonsson's saga (1217 to 1263). These sagas were written very shortly after the events they describe. However, as they don't overlap, we are given only one version of events (with the partial exception of the Bagler Sagas, which exist in two versions for the period 1202 to 1209), and this version tends to be from the viewpoint of the main character of the saga. From the later part of the period, fragments of documentation start to appear. The oldest Norwegian royal letter which is preserved was made out by Philippus the bagler king. Also, a couple of runic inscriptions written by central figures survive: A rune letter, probably written by King Sverre's son,